An organic semiconductor device can be produced under milder film-making conditions than those of a conventional inorganic semiconductor device, and formed into a thin film, or into a film at normal temperature, on various substrates. Therefore, it can be produced at a lower cost, and formed into a thin film on a polymer film or the like, allowing it to remain flexible.
Organic semiconductor materials under development include conjugated polymer compounds and oligomers thereof, e.g., polyphenylene vinylene, polypyrrole and polythiophene; and aromatic compounds centered by polyacene compounds, e.g., anthracene, tetracene and pentacene. In particular, it is reported that polyacene compounds can exhibit a high carrier mobility and excellent semiconductor device characteristics by virtue of high carrier mobility, because of their high crystallinity resulting from high intermolecular cohesion.
Polyacene compounds are applied to devices in the form of deposited film or single crystal. Their applicable areas under development include transistors, photovoltaic cells and lasers, among others (see Non-Patent Documents 1 to 3).
One of the methods other than deposition for forming a thin polyacene film spreads a solution of a precursor for pentacene as one polyacene compound on a substrate and heats it to form a thin pentacene film (see Non-Patent Document 4). A polyacene compound is not highly soluble in a solvent, and the above method uses a more soluble precursor to form a thin film by spreading the precursor solution and then converts the precursor into the polyacene compound under heating.
On the other hand, polyacene compounds with a substituent are discussed by Takahashi et al. (Non-Patent Document 5), Graham et al. (Non-Patent Document 6), Anthony et al. (Non-Patent Document 7) and Miller et al. (Non-Patent Document 8). Moreover, Non-Patent Documents 9 and 10 disclose synthesis of 2,3,9,10-tetramethylpentacene and 2,3,9,10-tetrachloropentacene, respectively.
An organic semiconductor material exceeding pentacene in mobility is not known so far.
However, the method for forming a thin polyacene film by use of the precursor of polyacene compound, e.g., that described above, needs treatment at high temperature to convert the precursor into the polyacene compound (for example about 150° C. in the case of pentacene). Moreover, it is difficult to completely convert a precursor into a corresponding polyacene, with the result that the unreacted precursor may remain as a defect or modified into a defect at high temperature.
On the other hand, Takahashi et al. proposes various polyacene derivatives with one or more substituents. They are however silent on their characteristics as organic semiconductor materials and making their thin films. Moreover, 2,3,9,10-tetramethylpentacene and 2,3,9,10-tetrachloropentacene are synthesized. However, their thin films exhibit a lower mobility than that of pentacene. In particular, the latter is modified in a film-making process carried out at high temperature to fail to exhibit semiconductor properties.
Still more, polyacene compounds tend to lose oxygen or water stability as their number of condensed rings increases. Pentacene, for example, is relatively stable when it is solid, but may be readily oxidized with oxygen into polyacenequinone when it is in the form of solution. This trend is more noted when it has an electron donating group, e.g., alkyl group, at the end of the major axis. Polyacenequinone exhibits no properties as a semiconductor, and a polyacene compound preferably has high oxidation resistance to prevent degradation of properties.
It is an object of the present invention to solve the above problems involved in the conventional techniques and provide an organic semiconductor material exhibiting a high mobility, solubility in solvents and oxidation resistance. The other objects are to provide an organic semiconductor thin film exhibiting a high mobility, and an organic semiconductor device excellent in electronic characteristics.    Non-Patent Document 1: “Advanced Materials,” 2002, vol. 14, p. 99    Non-Patent Document 2: Dimitrakopoulos et al., “Journal of Applied Physics,” 1996, vol. 80, p. 2501    Non-Patent Document 3: Cloak et al., “IEEE Transaction on Electron Devices,” 1999, vol. 46, p. 1258    Non-Patent Document 4: Brown et al., “Journal of Applied Physics,” 1996, vol. 79, p. 2136    Non-Patent Document 5: Takahashi et al., “Journal of American Chemical Society,” 2000, vol. 122, p. 12876    Non-Patent Document 6: Graham et al., “Journal of Organic Chemistry,” 1995, vol. 60, p. 5770    Non-Patent Document 7: Anthony et al., “Organic Letters,” 2000, vol. 2, p. 85    Non-Patent Document 8: Miller et al., “Organic Letters,” 2000, vol. 2, p. 3979    Non-Patent Document 9: “Advanced Materials,” 2003, vol. 15, p. 1090    Non-Patent Document 10: “Journal of American Chemical Society,” 2003, vol. 125, p. 10190